1. Field of the Invention
The present invention concerns a method of characterizing an electrode binder. It further encompasses the binder selected by this method and the electrodes containing it.
2. Description of the Prior Art
A plasticized (non-sintered) electrode comprises a support serving as a current collector coated with a paste containing the powdered active material and a binder, to which a conductive material may be added.
The function of the binder is to assure the cohesion of the grains of active material to each other and to the support of the electrode prior to assembly of the storage battery and during its operation. The binder must have sufficient chemical stability vis a vis the components of the cell; the adhesive properties of some binders deteriorate upon cycling. The binder must also be capable of accommodating dimensional variations of the electrode during cycling throughout its service life.
Another function of the binder is to maintain the electrical contact between the grains of active material and to favor ion exchange with the electrolyte. The electrochemically active surface area of an electrode depends on the surface area wetted by the electrolyte. If the electrode is insufficiently wetted the active surface area is reduced, which leads to an increase in the local current density and a lower charged capacity. To promote wetting of the electrode by the aqueous electrolyte the binder must have a hydrophilic character. The surface area accessible to the electrolyte depends on how the grains of active material are coated and bound by the polymer. The polymer film must have discontinuities enabling electron exchange.
At present, choosing an appropriate binder is based on an electrochemical evaluation under real conditions of use. The low concentration of binder used in the fabrication of an electrode necessitates long-term tests to show up the differences in performance between electrodes including the various binders to be evaluated. Under these conditions, optimizing the composition of the electrodes is a slow and random process, which considerably limits the possibilities of improving the batteries.
An aim of the present invention is to propose a method of selecting a binder to be used in the composition of an electrode and conferring high performance on it throughout its period of use.
Another aim of the invention is to propose a fast and easy method of choosing an electrode binder.